Abridgment of losses in armored single-conductor lead-covered A-C. cables

Abstract

Besides the copper loss and the dielectric loss, the losses occurring in single-conductor armored cables, whose sheath and armor are bonded and grounded at more than one point, are the circulating-current losses in the sheath and in the armor, and the additional iron losses in an armor of magnetic material. The circulating-current losses are due to the currents induced in the sheath and armor circuits by the fluxes linking these circuits when these circuits are closed by the bonds or grounding connections. The sheath and armor losses have been analyzed for a steel-wire armored cable, a copper-wire armored cable, a steel-tape armored cable, and a cable enclosed in an iron pipe, and have been compared with the losses occurring in a plain lead-covered cable without armor. The data used include test data and calculated values. It is shown that a steel-wire armored 350,000-cir. mil single-conductor cable with sheath and armor short-circuited by low-impedance bonds had a total loss (exclusive of dielectric losses), of 2.8 times the conductor loss, by test in a single-phase 60-cycle circuit at 4 ft. cable spacing, at 260 amperes. The corresponding loss in a similar cable without armor, with lead sheath short-circuited, was 2.4 times the conductor loss. Thus the steel-wire armored cable had an additional loss due to the armor of about 20 per cent. A steel-tape armor or an iron pipe surrounding a single-conductor cable usually introduces considerably higher losses. Under the same condition, a cable similar to the steel-wire armored one but having an armor of copper wires has a calculated total loss (exclusive of dielectric loss) of only 1.3 times the conductor loss, the armor current being 92 per cent of the conductor current. The reduction of losses due to diminished cable spacing in a single-phase circuit is shown. When the cable spacing is reduced from 10 ft. to 1 ft. the over-all losses for a steel-wire armored 350,000-cir. mil cable (exclusive of dielectric losses) are reduced about 8 per cent, while those for a similar cable with copper-wire armor drop only about 3 per cent. The relative total annual costs of a steel-wire armored cable and of a similar copper-armored cable, both cables having a conductor cross-section of 350,000 dr. mils, were compared. In view of the lower operating cost of the copper-armored cable, it was found to have a lower total annual cost than that of the steel-wire armored cable at load factors above 60 per cent, the steel-wire armor having the advantage economically at lower load factors.